The invention relates to a packet switching telecommunications network comprising a plurality of switching nodes between which packets are intended to be transported by logic paths, said network comprising, for securing a logic path called main path between a first and a second switching node, means for linking at least another logic path called substitute path to said main path which substitute path is intended to transport the same packets, the second node comprising switching means for switching from a main path to a substitute path.
The invention also relates to a switching node intended to be used in such a telecommunications network.
The invention has advantageous applications in the field of telecommunications network security, more particularly, for networks of the ATM type.
Networks of the ATM type use the idea of virtual paths which makes it possible to handle a total of up to 216 virtual circuits. More particularly, it is thus possible to secure the network by duplicating certain of these paths. Such a solution is described in Canadian Patent Application no. 2,132,063 filed by Siemens on Sep. 14, 1994.
This Patent Application describes indeed the use of pairs of paths formed by a main path and a substitute path which use different routes in the network between a first and a second switching node of the ATM network. The cells arriving at said first switching node are duplicated to be transmitted to said second switching node by the main path and by the substitute path. In normal operation, only the cells arriving by the main path are transmitted by the second switching node, whereas the cells arriving by the substitute path are suppressed. On the other hand, in the case of malfunctioning of the main path, it is the cells arriving by the substitute path which are transmitted by the second switching node, whereas the cells arriving by the main path are suppressed.
However, for this security to be effective it is necessary that the main path and the substitute path use different routes as much as possible. This implies different network transit times on either one of these paths. By way of example, for a high-rate network that utilizes transmission means based on optical fibers, a 100 km route difference between two paths causes a shift of the order of 1 ms. For cells of 53 octets (size of the ATM cell) and for virtual paths having a rate of several Mbit/s, such a shift is likely to cause the loss or the duplication of about ten cells when a switch is made from a main path to a substitute path.
Such a loss or duplication of cells leads to the disturbance of most of the services carried by the respective virtual path, more especially as the cells coming from the various services are generally interleaved to a maximum level in order to ensure a uniform rate for the various services. The upper layers of the disturbed services when an irregularity is established will make requests for return transmissions, which will have a highly undesirable xe2x80x9csnowballxe2x80x9d effect especially in the case where the cause of the switching from the main path to the substitute path was just a congestion of the main path.
It is an object of the invention to propose a system which remedies this disadvantage.
Therefore, a system according to the invention and as described in the opening paragraph is characterized in that the second node comprises calculation means for calculating a shift between the packets received on said paths, and check means for checking the switching to avoid packet losses or packet duplications by taking said shift into account.